Preheat the oven to 180°C (160°C fan / 350°F). Grease a 19 x 19 x 3.5 cm baking pan with butter or line it with greaseproof paper.

Separate the 3 large eggs. Place the egg yolks in a mixing bowl with ½ cup golden erythritol (100 g). Whisk with an electric mixer for about 2 minutes, until the mixture is lighter in color.

Add 3.5 tablespoons melted butter (50 g), ¼ cup Greek yoghurt (62 g), 1 teaspoon vanilla, and ⅓ cup almond milk (80 ml) to the yolk mixture. Whisk until combined.

In a separate bowl, stir together the dry ingredients: 2 tablespoons coconut flour (16 g), 1½ cups almond flour (150 g), 1 tablespoon cacao (6 g), 2 teaspoons baking powder, and a good pinch of salt.

In a clean bowl, beat the egg whites with an electric mixer until soft peaks form (fluffy but not stiff).

Add the dry ingredients to the wet ingredients and whisk just until combined. Do not overmix.

Gently fold the beaten egg whites into the batter with a spatula until mostly uniform and no large streaks remain.

Pour the batter into the prepared pan and level the top with a spatula. Bake for 30–35 minutes, or until a skewer inserted into the center comes out clean or with minimal crumbs. Avoid overbaking; the cake will firm up as it cools.

While the cake bakes (or immediately after removing it from the oven), make the toffee sauce: in a saucepan combine 7 tablespoons butter (100 g), ⅓ cup golden erythritol (or allulose), and ½ cup heavy cream (110 g). Heat to melt, bring just under a boil, then reduce to a low simmer and cook for about 10–12 minutes, whisking a few times during cooking, until the sauce is golden and has thickened slightly.

Remove the sauce from heat and stir in ½ teaspoon vanilla extract. The sauce will thicken further as it cools.

Let the cake rest briefly (about 10 minutes) so it firms slightly, then slice. Reheat individual slices if desired, spoon the hot toffee sauce over each slice, and serve.